1. Field of the Invention
The present invention relates to a current mirror circuit for generating a current which is in a constant ratio to a reference current.
2. Description of the Background Art
In general, semiconductor integrated circuits often employ a current mirror circuit for generation of a current which is in a constant ratio to a reference current (including a current equivalent to the reference current). FIG. 6 is a circuit diagram illustrating the construction of a current mirror circuit CM2. The current mirror circuit CM2 comprises two N-channel MOS transistors T1 and T2 having sources connected to each other and gates connected to each other, with the drain of the N-channel MOS transistor T1 connected to the gate thereof.
In the current mirror circuit CM2, since the N-channel MOS transistors T1 and T2 are at the same gate potential, a reference current Iref supplied to the drain of the N-channel MOS transistor T1 provides a constant ratio of the value of the reference current Iref to the value of a current Iout flowing between the drain and source of the N-channel MOS transistor T2. This ratio may be controlled depending on a size ratio between the N-channel MOS transistors T1 and T2.
The current mirror circuit CM2, of course, may employ P-channel MOS transistors. Also, bipolar transistors may be used in place of the MOS transistors, in which case the sources, drains and gates of the above described MOS transistors should be replaced with emitters, collectors and bases, respectively, for connections therebetween. In such cases, similar size adjustment may be made to generate the current Iout which is in a constant ratio to the reference current Iref.
Such a current mirror circuit is based on the precondition that it operates in a so-called constant current region (referred to as a saturated region for a MOS transistor or as an unsaturated region for a bipolar transistor) which is found in the relationship between a drain-source voltage and a drain-source current for a MOS transistor or the relationship between a collector-emitter voltage and a collector-emitter current for a bipolar transistor.
Unfortunately, the bipolar transistor presents the problem of the Early effect such that the increase in a base-collector voltage changes the width of a depletion layer between the base and the collector to change a substantial base layer width. As the collector-emitter voltage increases, the collector-emitter current is not constant but slightly increases because of the Early effect. Thus, the change in the collector-emitter voltage causes the change in the collector-emitter current even in the so-called constant current region, although a base-emitter current is constant.
Similarly, also in the MOS transistor, as the drain-source voltage increases, the drain-source current is not constant but slightly increases even if a gate-source voltage is constant. This results from the change in a channel length and is described using a value known as a channel length modulation parameter.
For this reason, for example, in the above described current mirror circuit CM2, when the drain-source voltage of the N-channel MOS transistor T2 varies, the value of the output current Iout is changed. This might fail to provide the constant ratio between the value of the reference current Iref and the value of the output current Iout which should be determined only by the transistor size ratio.